1. Field of the Invention
This invention relates to filters and, more particularly, to an anti-rotation device for a replaceable, spin-on type of oil filter particularly suited for use on an aircraft.
2. Background Art
For certain uses of a filter or pressure vessel, added assurance may be needed that same will remain intact, such as where the filter will experience excessive vibration. For example, airplane spin-on oil filters experience vibration and require special precautions to prevent the filter from spinning off or otherwise becoming loose. Should the filter become loose, oil could leak, oil pressure could drop and the engine could suffer catastrophic damage, resulting in a very dangerous situation. To prevent this dangerous situation, aviation filters are required by federal regulations to accept an additional lock or safety wire extending between the filter and the filter mount.
One effort to address this requirement has included welding a metal member to the closed or "dome" end of a cylindrical metal filter. The metal member has four spaced tabs, each including a hole therein. See, e.g., FIG. 1 of U.S. Pat. No. 4,992,166. The holes receive a wire (or wires) that extends through tie downs attached to the mount, and the wire ends are tied off. By using this wire lock, rotation of the filter, and the resultant leakage, should be prevented. Also, the costs associated with this welding procedure are relatively reasonable, especially when compared with other prior art procedures described below.
A drawback associated with this structure is that the welding requires extra steps in filter manufacture. That is, the metal member must be separately formed and welded to the dome end of the otherwise complete filter. As the metal member must be welded, there is always the possibility that the welding process can perforate the housing, violating the integrity of the housing. This causes defects during manufacture or worse, a dangerous situation while in use. Also, if the weld becomes unstable in use, a dangerous condition can result.
Alternatively, the filter housing can be molded entirely of plastic and tabs molded integrally to project from the dome end of the filter to receive the wires. See e.g., FIG. 10 of U.S. Pat. No. 4,992,166.
The following drawbacks may be associated with this all plastic design. Molding the entire housing of plastic requires a relatively expensive mold. Dropping the filter could lead to breakage of the integral, projecting tabs.
Also, the above-referenced attempts at providing a reliable aviation filter lock down mechanism use holes located at the dome of the filter. The distance from the dome to the tie downs on the mount, however, is rather long, perhaps resulting is some rotation nonetheless, which might cause oil leakage, depending upon the quality of the gasket and its installation relative to the filter and the mount.
Still another way to provide an aviation filter lock down is to form a ring out of aluminum material, and connect the ring to the "open" end of a cylindrical metal filter housing using electromagnetic forces. This process is known as "Magneform," which is believed to be a trademark of Gulf General Atomic Inc. of San Diego, Calif. More particularly, the ring is formed with very close tolerances and, under electromagnetic forces, is fit onto the filter: the ring effectively shrinks to take the shape of the filter housing open end outer diameter. The fit is so tight that the ring does not rotate, thereby preventing rotation of the filter when the filter is mounted, and a wire is extended through holes formed in the ring.
Drawbacks to this Magneform approach include the need for expensive electromagnetic equipment, possible exposure of employees on the assembly line to dangerous electromagnetic radiation, high material costs, and the overall manufacturing/assembly process is somewhat complicated, slow and expensive. Further, because aluminum is relatively soft, there may be a tendency for the wire to begin cutting through the holes formed in the aluminum ring with extended use, thereby causing a dangerous condition.
As can be seen, the prior art still does not provide the most cost-effective, reliable, and easy to manufacture anti-rotation device for a pressure vessel or filter.